It is well known to coat silver halide photographic materials on cellulose acetate supports. In certain instances, it has been found advantageous to coat these materials on polyester supports when increased dimensional stability or mechanical strength of the photographic element is desired, as described in U.S. Pat. No. 3,649,336. In particular, it has been found that a polyethylene naphthalate ("PEN") support has excellent mechanical strength and curl relaxation characteristics compared to other supports. However, it is more difficult to obtain the required adhesion characteristics when coating aqueous-based photographic compositions on these polyester films, in contrast to the conventionally employed cellulose acetate based support, as noted in U.S. Pat. Nos. 5,292,628 and 4,116,696 and European Patent Publication EP 035,614.
It is well-known to apply to a support one or more subbing layers followed by the direct coating of a photographic layer in order to improve the adhesion of a subsequent layer.
It is also well-known to improve the adhesive strength between a layer adjacent to a support and the surface of the support by way of a surface treatment. Examples of these surface activation treatments include, but are not limited to: a chemical treatment, a mechanical treatment, a corona discharge, a flame treatment, a UV irradiation, a radio-frequency treatment, a glow discharge, an active plasma treatment, a laser treatment, a mixed acid treatment or ozone-oxidation. Such treatment may be employed with or without the application of a subbing layer. With a polyester based support, even the additional application of a polymer subbing layer has failed to provide the desired degree of adhesion.
If the adhesion between the photographic layers and the support is insufficient, several practical problems arise. If the photographic material is brought into contact with a sticky material, such as splicing tape, the photographic layers may be peeled from the support resulting in a loss of image-forming capability. In the manufacturing process, the photographic material is subjected to slitting or cutting operations and in many cases perforated holes are punched into the material for film advancement in cameras and processors. Poor adhesion can result in a delamination of the photographic layers from the support at the cut edges of the photographic material which can generate many small fragments of chipped-off emulsion layers which then cause spot defects in the imaging areas of the photographic material.
The foregoing property may be referred to as "dry adhesion". This property may be distinguished from "wet adhesion" which refers to the tendency of a photographic element to delaminate during wet processing of exposed film. The element may undergo spot delamination or blistering due to processing at elevated temperatures or may be damaged by transport rollers during processing or subsequent thereto.
In U.S. Pat. No. 4,116,696, improved dry adhesive strength between a polyethylene terephthalate support and a photographic layer was obtained using a subbing layer containing a hydrophilic resin and droplets of a nonvolatile or low volatile hydrophobic liquid, which is not completely miscible with the subbing layer composition, having a boiling point above about 120.degree. C. and a solubility in water of about 10 g/100 g water or less at 25.degree. C. Thus, this improvement was obtained by altering the composition of the subbing layer which is coated directly on the polyester support, as opposed to the present invention, which involves the composition of the bottom-most photographic layer, coated above the subbing layer.
Furthermore, U.S. Pat. No. 4,116,696 discloses hydrophobic liquids having a solubility in water of about 10 g/100 g water or less. This encompasses a vast number of hydrophobic liquids which vary widely in terms of their chemical properties. There is no differentiation among solvents in this patent, and thus it is not apparent from this reference that different solvents can have vastly different effects on the adhesion to the support. In the present invention, we have found that only a limited number of high boiling organic solvents are effective at promoting wet adhesion of polyester supports and the use of these materials to improve "wet" as opposed to "dry" adhesion has not been suggested.
U.S. Pat. No. 5,292,628 teaches that improved wet adhesion of photographic layers to a polyester film base is provided with a layer containing an oil-in-water emulsion consisting of oil-formers, colloidal silicon dioxide, and gelatin. Again, the solution to the adhesion problem involves improved subbing layer technology, as opposed to formulation of the bottom-most photographic layer over a subbing layer as described in the present invention. The patentee notes that "both the high oil-former content and the presence of colloidal silicic acid is a condition for adhesion improvement." In addition, the high-boiling organic solvents employed cover a very wide range of materials including many solvents ineffective in accordance with the invention.
U.S. Pat. No. 4,495,273 describes a color photographic element coated on cellulose triacetate support with improved mechanical properties. Dry adhesion between the photographic layers and the support is increased using a combination of droplets of a water-immiscible high boiling organic solvent and an adhesion promoting quantity of a vinyl addition polymer latex in the antihalation layer of the photographic element. The acetate support is a wholly different class from polyester. Again, the solvents are taught without regard to the recognition that most of the solvents are not useful for improving wet adhesion. Further, the patentee also requires the presence of a vinyl addition polymer latex which is not essential in the present invention.
Methods of improving adhesion to polyester supports include altering the subbing layer, which is usually a very thin layer containing very low levels of gelatin, making it difficult to incorporate effective quantities of an adhesion promoting solvent. Also, substantial additions to this layer cause its thickness to be increased, which can result in the subbing layer being less effective at promoting adhesion of subsequently applied photographic emulsion layers. Increased thickness is also not desirable due to other system constraints such as providing a maximum number of exposures in a cartridge.
The problem to be solved is to provide a photographic element having a polyester support which has improved wet adhesion of the hydrophilic photographic layers to the polyester support so that the film layers will remain in tact during the wet processing of the film.